1. Field of the Invention
This invention relates to the field of hydraulic systems for controlling operation of friction elements of an automatic transmission. More particularly, the invention pertains to an hydraulic system that produces multiple relationships between the pressure in a friction element and electrical current supplied to a solenoid-operated valve.
2. Description of the Prior Art
Automatic transmissions for automotive vehicles usually include a planetary gear unit, which controls the ratio of input speed to output speed of the transmission. The gear ratio is switched by selectively braking, or holding against rotation, the sun gear, planet pinion carrier, or ring gear of the planetary gear unit, or by driveably connecting these elements to those of another gear unit. Clutch engagement and braking is effected usually by hydraulically-operated clutches or brakes, including brake bands. A selected gear is engaged by selectively holding against rotation or driveably connecting components of the gearset. In making these gear ratio changes, variable force solenoids are often used to control the magnitude of hydraulic pressure in the ongoing and offcoming friction elements.
Generally, variable force solenoids operated in conjunction with a regulator valve to produce one predetermined relationship of hydraulic pressure in the friction element to the magnitude of electrical current supplied to the solenoid that controls the position of the regulator valve. However, the magnitude of torque transmitted between ongoing and oncoming friction elements varies with a number of factors that include engine speed, throttle position, and gear ratios.